1) Field of the Invention
The present invention relates to a coated aliphatic polyester film having a membrane formed by coating an aqueous coating liquid. More specifically, the invention relates to a coated aliphatic polyester film which is degradable in the natural environment, has good blocking resistance and is excellent in the durability of membrane effect due to small coefficient of variation in the membrane thickness. Still more specifically, the invention relates to a non-charged aliphatic polyester film or a anticlouding aliphatic polyester film wherein the membrane is an antistatic layer or anticlouding layer, and the effects of these layers are immediately exhibited and have excellent durability.
2) Description of Related Art
The problem of waste plastics has received focused attention in recent years. Conventionally, wrapping materials prepared from general purpose resins and waste plastics such as agricultural films were discarded after use and disposed by incineration or landfilling. However, these plastic wastes develop high combustion heat in the incineration disposal, and has led to problems on the durability of the incinerator and on the public health due to generation of a toxic gas by incineration of plastics such as polyvinyl chloride. Further, when the waste plastics are used for land filling, plastic molded articles do not decompose as intact, semipermanently remain in the soil while holding their original shape as refuse. Consequently, the effect of waste plastics on the natural environment has become a serious problem.
In such environment, various degradable plastics which are composed of aliphatic polyester and can be completely decomposed under natural environment to the level of natural by-products, carbon dioxide and water, have been found and are now in the practical application step.
There are attempts to use these degradable aliphatic polyesters in the form of films or sheets according to its mechanical strength, transparency and flexibility to various uses such as wrapping materials, magnetic recording materials, optical materials and general materials in industry.
In the aliphatic polyester, a lactic acid based polymer film which has lactic acid units in the molecular structure has no generation of fungi and transparency can be maintained. Thus, it is expected to be used for an external and internal cover of agricultural facilities, tunnel house films, multi-cultivation films and other agricultural films.
However, the aliphatic polyester films have the disadvantage of having high electrical insulation and tend to be electrostatically charged with ease. As a result, the operator has received an electric shock at the winding or slitting the film, running of the film has become unstable at the printing or other fabrication step, sticking has remarkably lowered workability and further, spark discharge has sometimes led to an ignition accident. When they are used as a product, adhesion of refuse or dust has also caused problem. Consequently, aliphatic polyester films have been desired to provide antistatic properties.
Generally, plastic films are provided with antistatic property by incorporating an antistatic agent such as a surface active agent into the resin.
The present inventors have applied the process to an aliphatic polyester film and obtained following results. Some kinds of antistatic agents can provide antistatic property. However, (1) because the antistatic effect is low, a large amount of incorporation is required and leads to reduction of mechanical strength of the film. (2) The antistatic agent is difficult to bleed to the film surface and a significant time is needed before emergence of the antistatic effect. (3) Durability of the antistatic effect is short. Due to the above and other additional problems, plastic films are still unsatisfactory as an industrial product.
Conventionally, cultivation of vegetables are carried out in a house of synthetic resin film such as polyvinyl chloride, polyethylene, and ethylene vinyl-acetate copolymer, in order to cultivate even in a low temperature season. In such case, the synthetic resin film is always required to transmit sunlight. Accordingly, development of cloudiness due to moisture on the surface of the synthetic resin film must be inhibited. A process for previously incorporating an anticlouding agent into the resin and a process for forming a membrane of the anticlouding agent on the surface of the film have been tried.
For example, Japanese Laid-Open Patent Sho 61-53038 has disclosed an agricultural polyethylene terephthalate film which has an anticlouding film layer on the surface of the agricultural polyethylene terephthalate film. However the agricultural polyethylene terephthalate film is insufficiently adhered to the anticlouding film layer. When used for a long time, the layer is peeled, separated and thus the effect cannot be maintained for a long time.